<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>monitor</title>
    <script src="vue.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="cityName">

    </div>

    <script>
        var vm=new Vue({
            el:'#app',
            data:{
                cityName:'shanghai'
            },
            watch:{
                cityName(newName,oldName){
                    console.log(newName,oldName)
                }
            }
        })
    </script>
</body>
</html>